Astrium and the German Aerospace Centre (DLR) have taken a decisive step towards the goal of mapping the world in 3D. On 14 October 2010, the TanDEM-X radar satellite moved into formation flight with its virtually identical twin TerraSAR-X. This transition into close formation means that the final preparation stage of the TanDEM-X mission has been reached, and routine operation is scheduled to start at the turn of the year. The objective of the mission is to produce a highly precise three-dimensional elevation model of the entire surface of the Earth.
Satellites flying in formation
Special tests were carried out to prepare both satellites for the now successfully completed transition to helix trajectory, in which the satellites orbit less than 400 metres apart. The satellites moved together in a three-day drift phase. Before the change in trajectory, TerraSAR-X and TanDEM-X had been flying one behind the other at a distance of 20 kilometres, which represents just three seconds in flight time. Now the two satellites are flying 'eye to eye'.
To achieve the necessary formation flight helix structure, TanDEM-X's orbit first had to be shortened by means of a trajectory manoeuvre so that the satellite could make up the 20-kilometre gap between itself and TerraSAR-X. With two further manoeuvres the distance between the satellites was then reduced to a mere 350 metres. During this approach phase, communication between the satellites could be tested for the first time.
Mapping mission: The TerraSAR-X and TanDEM-X satellites are gathering data in formation flight for a 3D elevation model of the Earth’s land surface.
The programme of tests on TanDEM-X is scheduled to last until the end of the year. After this, the satellites will start taking the pictures for the global elevation model, following a precisely planned strategy. As this will require absolutely parallel operation of the satellites over a period of three years, the scientists need to ensure in advance that their close-formation flight is optimised with the utmost accuracy for correct image capture.
In tandem, the satellites have stereo vision
Because the satellites are so close together, their two radar systems can be synchronised, enabling the satellites to operate together as a pair. In this 'bistatic' mode one satellite emits radar pulses for the picture and both then receive the signal reflected back from the ground.
In the same way as having two eyes means that we can see in 3D, parallel measurements by two satellites make it possible to produce stereoscopic maps of the Earth's surface. The data collected by the satellites in tandem will be used to produce the elevation model of the entire land surface of the Earth - a total area of 150 million square kilometres.
In contrast to the interferometric images previously generated from various separate satellite flights, the satellite duo - which together forms the first ever free-flying radar interferometer - will generate instantaneous parallel bistatic pictures of the Earth's surface.
In addition, TerraSAR-X and TanDEM-X can synchronise their radar instruments via an inter-satellite link, and TanDEM-X can also monitor the 'health' of its twin.
Ralf Zimmermann, Head of German Programmes and Applications, is very happy with the way the project has gone so far. "Both radar satellites are flying in virtually identical solar synchronous orbits - at a speed of 28,000 kilometres per hour - and at times are only 200 metres apart. This constellation means we will be able to perform a 'world premiere' in radar technology. For the very first time, a bistatic radar instrument will operate on two satellites, thus providing the data for a global three-dimensional model of the world with a precision never previously attained."
A fantastic future for data enrichment
One of the first bistatic pictures shows Mount Etna on the eastern coast of Sicily. (© DLR)
In order to derive highly detailed elevation information from satellite radar data, two pictures of the area being mapped are always needed. The closer the two images are to each other (in terms of time picture taken, image angle, and satellite orbit), the more precise the mapping results will be.
The raw data provided by the two satellites will be enriched by Infoterra to meet the different requirements of a whole range of customers. Looking to the future, Andreas Kern, Head of Business Development and Sales, says: "With the availability of the homogeneous global elevation model in 2014, we will be in a truly unique position. No other company anywhere in the world will be able to compete with our data, and so the prospects for growth are highly promising." Infoterra is already supplying high-resolution all-weather radar data of unprecedented quality, and providing customers with reliable direct access at all times to TerraSAR-X data.
The sectors in which the TanDEM-X elevation model might be applied in future are many and varied; these include oil and gas field management support, improved crisis management and anticipation, forecasting the effects of disasters, and the targeted preparation of defence and security assignments.
The TanDEM-X mission is being managed for the German Aerospace Centre (DLR) with funding from the German Ministry of Economics and Technology under a Public-Private Partnership (PPP) with Astrium GmbH. Astrium built the satellite and also shares the development and operation costs. DLR is responsible for scientific use of the TanDEM-X data, planning and managing the mission, controlling the two satellites and generating the digital elevation model. As with TerraSAR-X, Infoterra GmbH, part of Astrium Services, will manage the commercial marketing of the TanDEM-X data.
• Vertical accuracy: 2m (relative) 10 m (absolute)
• Horizontal grid: 12 x 12 m
• Global homogeneous mapping for the first time, without interruptions at regional or national frontiers
• Completely reliable data acquisition
• 2014: Three-dimensional digital model of the Earth's entire surface